I. Field of the Invention
This invention relates generally to wastewater treatment systems which physically and biologically treat wastewater and, more particularly, to treatment systems which use floating filter media to physically and biologically treat wastewater.
II. Description of Prior Art
In high density aquaculture systems used for the production of aquatic animals such as catfish, tilapia, alligators, and clams, and in other water systems which generate wastewater, it is necessary to remove suspended solids, optimize nitrification and to reduce biochemical oxygen demand (BOD) of the wastewater. In all cases, treated water is eventually returned to the ecosystem, whether the return is to a lake or stream or back to a controlled aquaculture system. Varying load and flow conditions make operation of waste treatment systems difficult, in that the timing of treatment steps is dependent upon such changing conditions.
In the operation of any wastewater treatment system, it is desirable to waste as little water as possible and to maximize the concentration of waste sludge accumulated by the system. This is especially true in high density aquaculture systems, where constant recirculation of water is necessary, and it becomes even more important to minimize the amount of water wasted in the treatment process. Water loss also becomes critically important when the waste water volume approaches the volume of the aquaculture system being treated, such as in aquaria used in pet stores and research labs. The principal problems with high water loss in any aquaculture system are: 1) the costs of treating high volumes of the backwashed waters, 2) in warm water aquaculture systems, heat losses due to release of the backwash water, 3) increased water demands and pretreatment costs, 4) high capital costs for pumps and other equipment whose size is dictated by peak water demands, and 5) high costs of replacing synthetic salts lost through backwashing a system being used in a marine (i.e. saltwater) application.
Most prior art systems accomplish treatment using various traditional treatment components, such as aeration basins, filters and clarifier units, with each component having its own treatment and energy consumption limitations. A system which efficiently combines the features of separate component systems would have greater advantages over those presently in use.
Three known filters which overcome most of these problems are the biofilters disclosed in U.S. Pat. No. 5,126,042, U.S. Pat. No. 5,232,586, and U.S. Pat. No. 5,445,740, all issued to the inventor herein, Dr. Ronald F. Malone (hereinafter xe2x80x9cMalone Ixe2x80x9d, xe2x80x9cMalone IIxe2x80x9d, and xe2x80x9cMalone IIIxe2x80x9d respectively). Malone I uses a tank having sidewalls which are inwardly sloping toward the bottom, wherein a floating media pack is caused to form near the top of the tank when it is filled with liquid during filtration. A high-speed, propeller-type agitator is employed to fluidize and expand the media pack and break up the filtered matter prior to backwashing the system. A drain line is opened near the bottom of the tank to allow accumulated sludge to leave the tank, and an outlet line is provided immediately above the media pack. Malone III is similar to Malone I, but uses slowly rotating paddles to gently fluidize and expand the media pack rather than the high speed propellers of Malone I.
Malone II employs a tank having an upper filtration chamber and a lower expansion chamber fluidically connected to each other by a constricted passageway. An inlet line supplies water to the tank through the lower chamber, while a floating media pack forms within the upper chamber during filtration. As in Malone I, an outlet line is connected to the tank above the media pack and delivers filtered water back to the aquatic environment. Contrary to Malone I, however, backwashing is accomplished by the displacement and expansion of the media pack through the constricted passageway. The turbulence of this expansion causes the filtered matter and sludge to fall toward a drain line located at the bottom of the tank. In most embodiments of Malone II, the backwashing method results in large water losses as compared to the methods of Malone I and III.
While these devices are well-suited to accomplishing the objectives stated in those patents, there still remain certain disadvantages inherent is these and other prior art systems. Periodic backwashing is a necessity for all expanding media filters and in many applications 5 or 6 backwashes per day is recommended. The prior art generally requires water to be flushed or removed from the tank during each backwashing operation. The backwashing water generally exits through a sludge line, is intermixed with the sludge and must be treated by a separate process before being released into the environment. This results in water being lost from the system under filtration and higher costs incurred in disposing of sludge mixed backwash water. As alluded to above, this water loss is particularly costly where the biofilter is used in a marine application since marine applications often employ water reconditioned with synthetic salts. In many instances, the costs of synthetic salts may make the replacement of reconditioned water lost in backwashing cost prohibitive. For example, most of the Malone II type biofilters may lose between 10 and 15 gallons of water for every cubic foot of filter media contained in the biofilter. While Malone I and III backwash with less water loss than Malone II (2-5 gallons per cubic foot of filter media), Malone I and III generally employ metal components in the structure supporting the propellers or paddles. In a marine environment, this structure is highly susceptible to corrosion by the salt water.
Furthermore, the prior art biofilters typically require the opening and closing of different valves during the backwashing process. For example, the influent and effluent lines are general closed and the sludge line opened during backwashing operations. While this is often automated to avoid tedious manual operation of the valves, the automating equipment is a significant part of the total cost of the biofilter. Automation of backwashing also includes the risks associated with a failure of the automating equipment. For example, automated backwashing systems generally include an automated ball valve. If the ball valve fails in the open position, it is possible that the whole system could be siphoned out through the sludge line. Finally, prior art systems like Malone I and III required motorized equipment to fluidize the filter media during backwashing, further adding to the costs of producing these types of biofilters.
Because of the energy required to backwash and the loss of water occurring during each backwashing operation, there is a practical limit in the prior art on how often a biofilter may be backwashed. In turn, less frequent backwashing leads to other problems and disadvantages. Less frequent backwashing allows more solids to accumulate in the filter media and adversely impact the nitrification rate of ammonia. Since ammonia is toxic to fish life, it is important in aquaculture systems to reduce the NH4 and NH3 (collectively known as Total Ammonia Nitrogen or TAN) to nitrite (NO2) and nitrate (NO3). As these solids decay, they both produce ammonia and consume the oxygen which could otherwise be used to reduce ammonia to nitrite and nitrate. Decaying solids also encourage growth of heterotrophic bacteria which compete for space with more desirable autotrophic bacteria. Additionally, the accumulation of solids and the overgrowth of biological floc in the filter media caused by less frequent backwashing increases the influent pressure required (and therefore the pumping capacity required) to force water through the filter media. This head loss caused by insufficiently frequent backwashing also prevents the employment of more efficient pumping technologies such as air lift pumps. Air lift pumps are far more economical to build and operate than convention pumps, but have limited lifting capacity making them impractical for use with prior art biofilters. However, if a system was developed which rendered frequent backwashing economical, the head loss across the filter media could be kept sufficiently low so that air lift pumps could be effectively employed in biofilters.
Another disadvantage found in prior art biofilters is the tendency to experience xe2x80x9cblow down turbidity.xe2x80x9d Blow down turbidity occurs at the end of the backwash cycle and is a result of the fluidization of sediments during backwashing. When the biofilter is returning to the normal filtration stage, a certain amount of turbid water is forced through the effluent outlet before the media can re-form into a compact enough bed to effectively filter the entrained solids.
Finally, certain characteristics of the prior art required the biofilter tank to take on special geometric shapes in order for the biofilter to operate in the most efficient manner. For example, a preferred embodiment of the biofilter in Malone II taught a restriction in the midsection of the tank in order to most efficiently expand the filter media during backwashing.
Therefore, it is an object of this invention to provide a floating media biofilter which allows backwashing of the media without the loss of water.
It is another object of this invention to provide a floating media biofilter that will allow considerably more frequent backwashing than practical with prior art biofilters.
It is a further object of this invention to provide a floating media biofilter that does not require electro-mechanical valves or other components that are subject to failure.
It is still a further object of this invention to provide a floating media biofilter that is more economical to built and operate than hereto known in the art.
Therefore the present invention provides a tank for a floating media biofilter. The biofilter tank comprises a filter chamber and a charge chamber for intermittently storing air. The charge chamber includes an air outlet for admitting air into said filter chamber, a water inlet, an air inlet, and a trigger for selectively allowing the passage of air through said air outlet.